If your focus is solely on the postures during yoga exercises, you may miss out of the benefits conferred by the other pillars of a whole practice: Proper rest, proper breathing, proper diet, and positive thinking.
Proper rest takes place not only in savasana at the end of your exercises, but always. This means we rest the parts of the body that are holding onto unnecessary muscular tension. So often we uphold a state of tension in readiness for work, that there is very little energy left to do the actual work!
So we rest the body by softening around tension, we rest the mind by observing the quality of your breath, we rest the spirit by letting go of body identification to dwell in simple awareness.
Proper breathing takes place when we relax the abdomen and allow our inhales to fill up the lower, then mid, then upper lungs. During the exhale we empty from the top of the lungs, down to the lower lungs. It is common to breathe shallowly from the top of our lungs, particularly during moments of intense emotion or stress.
The point is to expel old, foul, toxic air from all the air sacs of the lungs, places ripe for bacteria to grow, and replace it with fresh, vital energy that rides in on your breath.